This invention relates to phase monitor devices designed to protect electrical equipment powered by three phase voltage changes in phase and voltage.
The prior art contains numerous phase monitoring devices. Unfortnately, these prior phase monitors are very limited in the forms of protection they provide. For example, many prior phase monitors do not provide for over voltage protection or for hysteresis, sometimes called "dead band" of the setpoints. Also, prior phase monitors are very inefficient and require highere power to operate them, thereby increasing the operating temperatures of the device which results in a decrease in the life of the instrument.
On the other hand, the instant phase monitor circuitry provides for lower power dissipation which extends the operating temperature range and life of the instrument. Also the addition of hysteresis further protects the controlled load by preventing the load from being cycled "on" and "off" by slightly varying voltages at the lower or upper limits. The instant circuitry is designed so that after the voltage has exceeded the lower or upper limit which disables the load, the voltage must return to approximately three percent (3%) within operational limits before the load is enabled again to run the protected equipment.
The instant design also incorporates an over voltage setpoint not found in many prior circuits. The setpoits are normally set at plus or minus ten percent (10%) of the voltage set on the devices adjustment control.
A further feature of the instant invention not found in prior circuitry is a switching-type power supply to drive the output relay coil which decreases power dissipation, contrary to circuits of prior existing phase monitor devices which power the output relay from rectified line voltage.
The instant circuitry also has another low dissipation power supply which powers the circuitry. This low voltage supply uses capacitive reactance which does not dissipate power rather than resistance like prior art circuitry, to derive the supply voltage for the electronics.